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# Ch03

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### Transcript

• 1. A First Book of ANSI C Fourth Edition Chapter 3 Processing and Interactive Input
• 2. Objectives
• Assignment
• Mathematical Library Functions
• Interactive Input
• Formatted Output
• 3. Objectives (continued)
• Symbolic Constants
• Case Study: Interactive Input
• Common Programming and Compiler Errors
• 4. Assignment
• The general syntax for an assignment statement is
• variable = operand;
• The operand to the right of the assignment operator (=) can be a constant, a variable, or an expression
• length=25; is read “length is assigned the value 25”
• Subsequent assignment statements can be used to change the value assigned to a variable
• length = 3.7;
• length = 6.28;
• 5. Assignment (continued)
• The operand to the right of the equal sign in an assignment statement can be a variable or any valid C expression
• sum = 3 + 7;
• product = .05 * 14.6;
• The value of the expression to the right of = is computed first and then the calculated value is stored in the variable to the left of =
• Variables used in the expression to the right of the = must be initialized if the result is to make sense
• amount + 1892 = 1000 + 10 * 5 is invalid!
• 6. Assignment (continued) If width was not initialized, the computer uses the value that happens to occupy that memory space previously (compiler would probably issue a warning)
• 7. Assignment (continued)
• = has the lowest precedence of all the binary and unary arithmetic operators introduced in Section 2.4
• Multiple assignments are possible in the same statement
• a = b = c = 25;
• All = operators have the same precedence
• Operator has right-to-left associativity
• c = 25;
• b = c;
• a = b;
• 8. Implicit Type Conversions
• Data type conversions take place across assignment operators
• double result;
• result = 4; //integer 4 is converted to 4.0
• The automatic conversion across an assignment operator is called an implicit type conversion
• answer = 2.764; //2.764 is converted to 2
• Here the implicit conversion is from a higher precision to a lower precision data type; the compiler will issue a warning
• 9. Explicit Type Conversions (Casts)
• The operator used to force the conversion of a value to another type is the cast operator
• (dataType) expression
• where dataType is the desired data type of the expression following the cast
• Example:
• If sum is declared as double sum; , (int) sum is the integer value determined by truncating sum ’s fractional part
• 10. Assignment Variations sum = sum + 10 is not an equation—it is an expression that is evaluated in two major steps
• 11. Assignment Variations (continued)
• 12. Assignment Variations (continued)
• 13. Assignment Variations (continued)
• Assignment expressions like sum = sum + 25 can be written using the following operators:
• += -= *= /= %=
• sum = sum + 10 can be written as sum += 10
• price *= rate is equivalent to price = price * rate
• price *= rate + 1 is equivalent to price = price * (rate + 1)
• 14. Accumulating
• The first statement initializes sum to 0
• This removes any previously stored value in sum that would invalidate the final total
• A previously stored number, if it has not been initialized to a specific and known value, is frequently called a garbage value
• 15. Accumulating (continued)
• 16. Accumulating (continued)
• 17. Counting
• A counting statement is very similar to the accumulating statement
• variable = variable + fixedNumber;
• Examples: i = i + 1; and m = m + 2;
• Increment operator (++): variable = variable + 1 can be replaced by variable++ or ++variable
• 18. Counting (continued)
• 19. Counting (continued)
• 20. Counting (continued)
• When the ++ operator appears before a variable, it is called a prefix increment operator ; when it appears after a variable, it is called postfix increment operator
• k = ++n; is equivalent to
• n = n + 1; // increment n first
• k = n; // assign n's value to k
• k = n++; is equivalent to
• k = n; // assign n's value to k
• n = n + 1; // and then increment n
• 21. Counting (continued)
• Prefix decrement operator: the expression k = --n first decrements the value of n by 1 before assigning the value of n to k
• Postfix decrement operator: the expression k = n-- first assigns the current value of n to n and then reduces the value of n by 1
• 22. Counting (continued)
• 23. Mathematical Library Functions
• 24. Mathematical Library Functions (continued)
• The argument to sqrt must be floating-point value; passing an integer value results in a compiler error
• Return value is double-precision
• Must include #include <math.h>
• 25. Mathematical Library Functions (continued)
• 26. Mathematical Library Functions (continued)
• 27. Mathematical Library Functions (continued) Argument need not be a single constant
• 28. Mathematical Library Functions (continued)
• The step-by-step evaluation of the expression
• 3.0 * sqrt(5 * 33 - 13.91) / 5
• is (see next slide)
• 29. Mathematical Library Functions (continued)
• 30. Mathematical Library Functions (continued)
• Determine the time it takes a ball to hit the ground after it has been dropped from an 800-foot tower
• time = sqrt(2 * distance/g), where g = 32.2 ft/sec2
• 31. Mathematical Library Functions (continued)
• 32. Interactive Input
• 33. Interactive Input (continued)
• This program must be rewritten to multiply different numbers
• scanf() is used to enter data into a program while it is executing; the value is stored in a variable
• It requires a control string as the first argument inside the function name parentheses
• 34. Interactive Input (continued)
• The control string passed to scanf() typically consists of conversion control sequences only
• scanf() requires that a list of variable addresses follow the control string
• scanf(&quot;%d&quot;, &num1);
• 35. Interactive Input (continued)
• 36. Interactive Input (continued) This statement produces a prompt Address operator (&)
• 37. Interactive Input (continued)
• scanf() can be used to enter many values
• scanf(&quot;%f %f&quot;,&num1,&num2); //&quot;%f%f&quot; is the same
• A space can affect what the value being entered is when scanf() is expecting a character data type
• scanf(&quot;%c%c%c&quot;,&ch1,&ch2,&ch3); stores the next three characters typed in the variables ch1 , ch2 , and ch3 ; if you type x y z , then x is stored in ch1 , a blank is stored in ch2 , and y is stored in ch3
• scanf(&quot;%c %c %c&quot;,&ch1,&ch2,&ch3); causes scanf() to look for three characters, each character separated by exactly one space
• 38. Interactive Input (continued)
• In printing a double-precision number using printf() , the conversion control sequence for a single-precision variable, %f , can be used
• When using scanf() , if a double-precision number is to be entered, you must use the %lf conversion control sequence
• scanf() does not test the data type of the values being entered
• In scanf(&quot;%d %f&quot;, &num1, &num2) , if user enters 22.87 , 22 is stored in num1 and .87 in num2
• 39. Caution: The Phantom Newline Character
• 40. Caution: The Phantom Newline Character (continued)
• The following is a sample run for Program 3.10:
• Type in a character: m
• The keystroke just accepted is 109
• Type in another character: The keystroke just accepted is 10
• 41. Caution: The Phantom Newline Character (continued)
• 42. Caution: The Phantom Newline Character (continued)
• 43. A First Look at User-Input Validation
• 44. A First Look at User-Input Validation (continued)
• As written, Program 3.12 is not robust
• The problem becomes evident when a user enters a non-integer value
• Enter three integer numbers: 10 20.68 20
• The average of 10, 20, and -858993460 is -286331143.333333
• Handling invalid data input is called user-input validation
• Validating the entered data either during or immediately after the data have been entered
• Providing the user with a way of reentering any invalid data
• 45. Formatted Output
• 6
• 18
• 124
• ---
• 148
Output is not aligned
• 46. Formatted Output (continued) 6 18 124 --- 148 Field width specifier
• 47. Formatted Output (continued)
• 48. Format Modifiers
• Left justification: printf(&quot;%-10d&quot;,59); produces the display 59 ٨٨٨٨٨٨٨٨
• Explicit sign display: printf(&quot;%+10d&quot;,59); produces the display ٨٨٨٨٨٨٨ + 59
• Format modifiers may be combined
• %-+10d would cause an integer number to both display its sign and be left-justified in a field width of 10 spaces
• The order of the format modifiers is not critical
• %+-10d is the same
• 49. Other Number Bases [Optional]
• The decimal (base 10) value of 15 is 15.
• The octal (base 8) value of 15 is 17.
• The hexadecimal (base 16) value of 15 is f.
• 50. Other Number Bases (continued)
• 51. Other Number Bases (continued)
• The decimal value of the letter a is 97.
• The octal value of the letter a is 141.
• The hex value of the letter a is 61.
• 52. Other Number Bases (continued)
• 53. Symbolic Constants
• Literal data refers to any data within a program that explicitly identifies itself
• Literal values that appear many times in the same program are called magic numbers
• C allows you to define the value once by equating the number to a symbolic name
• #define SALESTAX 0.05
• #define PI 3.1416
• Also called symbolic constants and named constants
• 54. Symbolic Constants (continued) # sign is a signal to a C preprocessor
• 55. Case Study: Interactive Input
• 56. Case Study: Interactive Input (continued)
• 57. Common Programming Errors
• Forgetting to assign initial values to all variables before the variables are used in an expression
• Calling sqrt() with an integer argument
• Forgetting to use the address operator, &, in front of variable names in a scanf() function call
• Not including the correct control sequences in scanf() function calls for the data values that must be entered
• Including a message within the control string passed to scanf()
• 58. Common Programming Errors (continued)
• Terminating a #define command to the preprocessor with a semicolon
• Placing an equal sign in a #define command when equating a symbolic constant to a value
• Using the increment and decrement operators with variables that appear more than once in the same expression
• Being unwilling to test a program in depth
• 59. Common Compiler Errors
• 60. Common Compiler Errors (continued)
• 61. Summary
• Arithmetic calculations can be performed using assignment statements or mathematical functions
• The assignment symbol, =, is an operator
• C provides the +=, -=, *= and /= assignment operators
• The increment operator, ++, adds 1 to a variable
• The decrement operator, --, subtracts 1 from a variable
• C provides library functions for calculating square root, logarithmic, and other mathematical computations
• 62. Summary (continued)
• Mathematical functions may be included within larger expressions
• scanf() is a standard library function used for data input
• When a scanf() function is encountered, the program temporarily suspends further statement execution until sufficient data has been entered for the number of variable addresses contained in the scanf() function call
• 63. Summary (continued)
• It is good programming practice to display a message, prior to a scanf() function call, that alerts the user as to the type and number of data items to be entered
• Field width specifiers can be included with conversion control sequences to explicitly specify the format of displayed fields
• Each compiled C program is automatically passed through a preprocessor
• Expressions can be made equivalent to a single identifier using the preprocessor #define command